248 



LOWER IX VER TEBRA TES. 



Branch VII. — 3I0LLUSCA. 



With the possible exceptious of the insects and the birds, there is no group in the 

 animal kingdom which is siich an universal favorite among all classes as the one now 

 under discussion. This is very natural; for the hard armor which they bear, and the 

 bright colors with which many of them are ornamented, renders them attractive, while 

 the comparative indestructibility of the same shells rendere the care of a collection an 

 easy task. But while the collectors of the shells are many, the real students of the 

 animals are few, and even now, although these forms have been collected and studied 

 by conchologists for many years, a satisfactory classification is still desired. 



The word MoUusca means soft, and it was applied by Linue to a group of animals 

 embracing of the true moUuscs only the naked forms, together with the hydroids, 

 echinoderms and annelids, while the shell-bearing molluscs were arranged as Testacea 

 in a section of his group of Vermes. Cuvier was tlie fii-st to introduce order into the 

 group. His studies during the seven years spent as tutor on the Xomiandy coast 

 resulted in a classification of the MoUusca upon truly scientific grounds. The group, 

 as recognized by him, embraced, besides the forms now admitted, the barnacles, the 

 ascidians, and the brachiopods, tnily a heterogeneous assemblage. In after years the 

 Polyzoa were drawn in. The first of these groups to be separated were the barnacles, 

 which were shown by Thompson to be Crustaceans in 1831. Then Kowalewsky, in 

 186-5, described the embryology of the ascidians, from wldch it was apparent that they 

 had no relationships with the molluscs, but were rather to be classed with the verte- 

 brates, and lastly, the brachiopods were absolutely divorced from the group, taking the 

 Polyzoa with them. 



A concise definition of the ilollnsca is impossible. Here, as elsewhere, nature 

 refuses to be bound by strict rules, and the best we can do is to form a general concep- 

 tion which shall be true of the majority of forms, and which will, at the same time, be 

 loose enoueh to admit all. A mollusc, then, is a bilaterally symmetrical, unscgmented 

 animal, usuaUv covered with a univalve oc bivalve shell. It has a ventral, muscular 

 portion (the foot) well developed ; a symmetrical nervous system, consisting of a 



brain or supra-oesophageal ganglia, an oeso- 

 j>hageal commissure, and a secondary brain 

 beneath the throat. Most forms, in their 

 development, pass through a trochozoon 

 stage. 



In some forms, esjiecially in the gastero- 

 pods, the bilateral symmetry of the lx)dy is 

 more or less obscured, owing to what may 

 be called a torsion of the body, but, never- 

 theless, if we make allowance for this twist- 

 ing, it can readily V>e traced. In the young, 

 the segmentation of the body is fiequently evident, but it entirely disappears in the 

 adults, except among the chitons, where the elements of the shell and the gills are 

 metamerically repeated. 



The foot is a muscular process on the lower surface of the body, which is highly 



«'.. — :^. — IraiTar;. oi moiiui.:-, a. anu=; '-. braiit. cere- 

 bral gaziglion: /. fc»ox; ff, genital opening: A. bean: 

 i, i^ajal ganglion: /. 'lirer: m. njootfa; n, kidney: 

 p. pedal ganglion; r, Tisceral ganglion. 



